Changes in the active-power delivery of an energy generator as are normal with wind turbines lead to changes in the power system frequency in an energy distribution system if it cannot be ensured by controlling other power stations that as much energy is fed into the power system as is consumed by the connected loads. Due to the increase in proportion of wind turbines in the energy distribution system, the costs for compensating for the fluctuations of the feed-in power caused by wind power are rising for the power system operators.
A method for operating a wind turbine with an electric generator, which can be driven by a rotor, for delivering electric power to an electric power system are known (DE 100 22 974 A1). In this method, the power delivered by the generator to the power system is controlled or adjusted in dependence on the power system frequency of the electrical power system, the delivered power being decreased with a rise in the power system frequency. However, this is not a matter of providing control power and the measures proposed would also be unsuitable for this purpose.
In the normal operation of a power system without wind turbines, the adjustment does not present problems since only the load or the power consumed changes. However, the required control processes must be managed even in a power system with conventional power stations and with wind turbines. The situation is quite different when unpredictable incidents occur in a mixed power system with wind turbines, that is to say, for instance, a power failure or a voltage dip as a consequence of a ground fault or short circuit. The way they have hitherto been operated, wind turbines cannot provide any additional power in this case as can be done by conventional power stations in order to reduce in this way the primary control requirement of the power system operator. However, the wind turbine could be operated in a throttled manner, that is to say with a power which is below that which could be delivered by the wind turbine if it were set optimally to the existing wind conditions. However, this would have to be done whilst continuously taking into consideration the changing system conditions, locally and internationally, and in accordance with the requirements of the system operators but also with the wind conditions which are anything else but constant or even sufficiently accurately predictable.
In the case of an incident in the power system, for example, the primary control power must be rapidly activated in order to be able to counteract the decrease in frequency initiated by an incident before the frequency has dropped to a critical value. Apart from the fast activation, sufficient reserve power must also be kept available. In thermal power stations, the primary control power is kept available by throttling down the turbine inlet valves, the system operator ensuring that the throttling is only limited in time and that the existing secondary control power can replace the primary control power in time. From the point of view of energy, such slight throttling does not represent a loss since less fuel is consumed which is then available at a later time.
In a wind turbine, a control margin could also be provided by such “slight throttling” of the wind turbine so that the wind turbine is operated with a lower power than would be permitted by the wind conditions. Considered from the point of view of energy, however, this means that the available wind energy is not utilized when it would be possible, i.e. the wind passes the wind turbine unused in this operating mode.
Thus, this slight throttling of a wind turbine is a very costly measure since the unused part of the energy of the wind is not stored. In the case of a gas turbine, in contrast, primary energy is retained in slight-throttling mode and can be used at a later time.
In fact, the conditions in such mixed energy distribution systems are such that the wind turbines with their converter-fed systems do not contribute anything to the system stability and mass inertia of the power system when operating conventionally. Even if wind turbines with converters were to be operated with constant power at the power system this would even lead to an increase in the necessary control requirement which would have to be compensated for with conventional power stations with their primary control power.